Ischemic heart disease is a leading cause of death in North America and is predicted to become more prevalent as the population ages (Scroggins, 2001). Ischemia and reperfusion lead to myocardial injury through a variety of mechanisms. For example, ischemia and reperfusion profoundly affect mitochondria, and preservation of their integrity and function is critical to salvage (Borutaite et al., 1995; DiLisa et al., 1998; Kay et al., 1997; Ferrari et al., 1996; Kobara et al., 1996). Oxidative phosphorylation is transiently increased after reperfusion but then diminishes; pyridine nucleotides are lost from the mitochondria, and respiration through complex I is impaired; superoxide production is increased, possibly through retrograde electron flow through complex I; the permeability transition pore opens, associated with loss of calcium homeostasis; and cytochrome c is released (Borutaite et al., 1995; Duan et al., 1989; DiLisa et al., 1998; Piper et al., 1985; Becker et al., 1999; Halestrap et al., 1998). However, it is not clear whether these mitochondrial alterations are initiated by an intrinsic response to the low oxygen tension of ischemia or arise in part due to changes in the cytosol. Cytosolic alterations are known to include acidosis, increased inorganic phosphate, elevated calcium, and a rise in long-chain acyl coenzyme A.
In addition, a variety of signal transduction pathways, including that of MAP kinases, particularly c-Jun NH2-terminal kinase (JNK), and p38, are activated during myocardial ischemia and reperfusion. Previously it was shown that JNK translocates from cytosol to mitochondria in response to ischemia/reperfusion, and that in a model of metabolic inhibition in adult rabbit cardiomyocytes, inhibition of JNK is protective (He et al., 1999).
Ischemic preconditioning confers myocardial protection through a brief period of ischemia and reperfusion preceding the more sustained ischemia/reperfusion insult (Murry et al., 1986). Preconditioning is characterized by earlier recovery of mitochondrial function with more efficient resynthesis of ATP. Thus, it is clear from these diverse studies that ischemia and reperfusion activate cytosolic signals that target the mitochondria to modulate their response during ischemia and reperfusion, and furthermore, that preconditioning also involves signaling from cytosol to mitochondria.
Current therapies for ischemic heart disease are directed at the restoration of blood flow to the ischemic region. However, during reperfusion the heart undergoes further damage due in large part to the generation of reactive oxygen species (ROS), e.g., superoxide anion (Singh et al., 1995; Flaherty et al., 1988). Elevated ROS can be detected within minutes after the reintroduction of oxygen to ischemic tissues (Bolli et al., 1995). ROS have been shown to be key mediators of cellular and myocardial injury, with free radical scavengers attenuating the associated injury (Cesselli et al., 2001). Low levels of superoxide play a role in signaling pathways possibly contributing to preconditioning (Sun et al., 1996) and the development of hypertrophy (Ito et al., 1995). However, higher levels are detrimental, causing lipid peroxidation and apoptosis (Siwik et al., 1999; Halmosi et al., 2001). There are no current therapies for reperfusion injury.
Thus, what is needed is a method to inhibit ischemia and/or reperfusion injury.